Add PSCI node to enable multi-processor startup.

Note that not every 8994 device firmware supports PSCI,
and even if, then it can only start the cores and not
shut them down.
Signed-off-by: Konrad Dybcio <konradybcio@gmail.com>
Link: https://lore.kernel.org/r/20200623224813.297077-10-konradybcio@gmail.comSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>